1. Field of the Invention
This invention relates to a bushing used with the lead wire portion of tank type electrical equipment, such as power transformers or circuit breakers for extra high voltages, 500 kv and above, and wherein the tank is filled with an insulating material such as an insulating gas, an insulating oil or the like.
2. Description of the Prior Art
High voltage electrical equipment may be used in an environment where damage due to airborne pollutants such as salt and dust is high. These devices utilize bushings having long porcelain tubes which increase both the surface leakage distance and the ability to withstand the contaminated environment for connection to overhead aerial wires. When such electrical equipment is employed in a region having a high frequency of occurrence of earthquakes, for example in Japan, they are continually exposed to risks due to the earthquakes and are designed with emphasis on seismic strength. When the bushing is installed and the electrical equipment encounters an earthquake, the amplification of the earthquake experienced by the bushing is affected by the position of installation, the foundation and tank portions of the equipment, the mounting seat for the bushing, etc. The natural frequency of the bushing is determined by the relationship between the weight distribution and the rigidity of the bushing. If the frequency of an earthquake approximates or equals the natural frequency of the electrical equipment, then a resonant phenomenon is developed such that vibrations are amplified to a very large magnitude by each of the tank, bushing mounting seat, etc. These amplified vibrations are applied to the bushing. Such an amplified vibration may exceed the breaking strength of the bushing resulting in the breaking of the porcelain tube.
The greater part of the frequencies of an earthquake ranges generally from one to ten hertz. Bushings mounted on electrical equipment of the 220 kv and higher classifications may have a natural frequency less than ten hertz. This figure is identical to the frequency of earthquakes. With bushings having porcelain tubes having dimensions of up to five meters even the strongest earthquakes experienced in the past do not exceed the breaking strength of these porcelain tubes which have sufficient seismic strength. However, the use of environmentally resistive, long, porcelain tubes of the 500 kv and higher classifications results in a high probability of having the natural frequency of the bushing equal to a few hertz or less which corresponds to the frequency of seismic waves. It is therefore possible to break the environmentally resistive, long, porcelain tubes upon the occurrence of a great earthquake. Strenuous efforts have been made to improve the seismic strength of these bushings. When the 1,000 kv class is put to practical use, one method of increasing the seismic strength of the bushings is by reinforcing the bushing in three or four directions from its extremity by means of stay porcelain tubes or the like. In this case, the vibration of the stays becomes a chordal vibration and a phenomenon is developed which includes a superimposed vibration different from that of the bushing portion. Thus, it is difficult to analyze the seismic strength and the reliability of the bushing. Also, it is necessary to consider the flashover voltage due to contamination when the stay porcelain tube portions are connected in parallel. The adhesion of contaminants is different between the bushing and stay porcelain tubes. With the stay porcelain tubes being smaller in diameter they are generally apt to be contaminated. In any case it is necessary to determine the magnitude of the flashover voltage in the parallel state. In this respect the reliability is also reduced. The present invention eliminates the need for stay porcelain tubes.